Adoptive Cell Therapy
Adoptive immunotherapy is an established and evolving therapeutic approach. In the setting of allogeneic haematopoietic stem cell transplantation (HSCT), donor lymphocyte infusions (DLI) are frequently given to treat relapse of haematological malignancies. Tumour infiltrating lymphocytes (TILs) are effective in treating metastatic melanoma. Genetic engineering of T-cells greatly increases the scope and potency of T-cell therapy: T-cell receptor transfer allows targeting of intracellular cancer antigens, while chimeric antigen receptors (CAR) allow targeting of surface cancer or lineage specific antigens. Clinical responses have been observed with both approaches, and numerous further trials are underway.
Acute adverse events can occur following adoptive immunotherapy. Graft-versus-host disease (GvHD) is a common and serious complication of DLI. Administration of engineered T-cells has also resulted in toxicity. For instance, on-target off-tumour toxicity has been reported in native T-cell receptor transfer studies against melanoma antigens; T-cells re-directed to the renal cell carcinoma antigen carbonic anhydrase IX (CAIX) produced unexpected hepatotoxicity. Immune activation syndromes have been reported after CD19 CAR therapy. Finally vector-induced insertional mutagenesis results in a theoretical risk of lymphoproliferative disorders. The incidence and severity of these toxicities is unpredictable. Further, in contrast to a therapeutic protein or small molecules whose adverse events usually abate with the half-life of the therapeutic, T-cells engraft and replicate, potentially resulting in escalating and fulminant toxicity.
Suicide Genes
A suicide-gene is a genetically encoded mechanism which allows selective destruction of adoptively transferred cells, such as T-cells, in the face of unacceptable toxicity. Two suicide-genes have been tested in clinical studies: Herpes Simplex Virus thymidine kinase (HSV-TK) and inducible caspase 9 (iCasp9).
The herpes simplex virus I-derived thymidine kinase (HSV-TK) gene has been used as an in vivo suicide switch in donor T-cell infusions to treat recurrent malignancy and Epstein Barr virus (EBV) lymphoproliferation after hemopoietic stem cell transplantation. However, destruction of T cells causing graft-versus-host disease was incomplete, and the use of ganciclovir (or analogs) as a pro-drug to activate HSV-TK precludes administration of ganciclovir as an antiviral drug for cytomegalovirus infections. Moreover, HSV-TK-directed immune responses have resulted in elimination of HSV-TK-transduced cells, even in immunosuppressed human immunodeficiency virus and bone marrow transplant patients, compromising the persistence and hence efficacy of the infused T cells.
The activation mechanism behind Caspase 9 was exploited in the original iCasp9 molecule. All that is needed for Caspase 9 to become activated, is overcoming the energic barrier for Caspase 9 to homodimerize. The homodimer undergoes a conformational change and the proteolytic domain of one of a pair of dimers becomes active. Physiologically, this occurs by binding of the CARD domain of Caspase 9 to APAF-1. In iCasp9, the APAF-1 domain is replaced with a modified FKBP12 which has been mutated to selectively bind a chemical inducer of dimerization (CID). Presence of the CID results in homodimerization and activation. iCasp9 is based on a modified human caspase 9 fused to a human FK506 binding protein (FKBP) (Straathof et al (2005) Blood 105:4247-4254). It enables conditional dimerization in the presence of a small molecule CID, known as AP1903. AP1903 is an experimental drug and is considered biologically inert since it does not interact with wild-type FKBP12. However clinical experience with this agent is limited to a very small number of patients (Di Stasi, A. et al. (2011) N. Engl. J. Med. 365, 1673-1683; and Iuliucci, J. D. et al. (2001) J. Clin. Pharmacol. 41, 870-879). AP1903 is also a relatively large and polar molecule and unlikely to cross the blood-brain barrier.
In an alternative approach, executioner caspases can be activated by small molecules using a complex strategy which involves introduction of tobacco etch virus (TeV) proteolysis sites into Caspase 3 or 6 or 7 and co-expression with a split TEV protease which is recombined in the presence of rapamycin (Morgan et al (2014) Methods Enzymol. 544:179-213). This is an unsatisfactory strategy for a clinically useful suicide switch for a number of reasons: firstly three separate proteins are required which is highly complex: the modified caspase, and the two components of the split TeV protease respectively; secondly, TeV components are xenogeneic and likely immunogenic; finally, this strategy only activates protease sensitive caspase molecules which are downstream and less sensitive than apical caspases.
A suicide gene based on CID activation of FAS has been described (Amara et al (1999) Hum. Gene Ther. 10, 2651-2655). This also depends on this CID for activation, and since it does not directly activate the apoptosis cascade, escape (through FAS resistance) is possible.
A homodimerization system based on a standard pharmaceutical which replaces the need for an experimental CID would be an attractive alternative. However, no homodimerizing small molecule pharmaceuticals are available.
Other suicide genes have been proposed for instance full-length CD20 when expressed on a T-cell can render T-cells susceptible to lysis by the therapeutic anti-CD20 antibody Rituximab (Introna, M. et al. (2000) Hum. Gene Ther. 11, 611-620). Further suicide genes have also been described on this theme of antibody recognition, for example: RQR8 renders T-cells susceptible to CD20 but is more compact than the full-length CD20 molecule (Philip, B. et al. (2014) Blood doi:10.1182/blood-2014-01-545020); a truncated version of EGFR (huEGFRt) renders cells susceptible to lysis by anti-EGFR mAbs (Wang, X. et al. (2011) Blood 118, 1255-1263); and a myc epitope tag expressed on a cell surface leaves cells susceptible to lysis with an anti-myc antibody (Kieback et al (2008) Proc. Natl. Acad. Sci. U.S.A. 105, 623-628). A major limitation of these antibody dependent approaches is their dependence on bioavailability of a therapeutic antibody at high local concentrations to act. It is known for instance that lytic antibodies are not particularly effective against bulky disease and a limitation of antibody based suicide genes is that cells resident where high antibody concentrations are not reached would escape. Further, in certain situations: for instance a severe macrophage activation syndrome or cytokine storm induced by a CAR T-cells; the additional immune activation induced by a monoclonal antibody may be deleterious to the clinical situation activation of the suicide gene is trying to treat.
There is thus a need for an alternative suicide gene which is not associated with the disadvantages mentioned above.